U.S. patent number 10,889,876 [Application Number 15/767,345] was granted by the patent office on 2021-01-12 for non-heat treated wire rod having excellent cold workability and manufactured method therefor.
This patent grant is currently assigned to POSCO. The grantee listed for this patent is POSCO. Invention is credited to Yong-Kwan Heo, Ha-Ni Kim, Sang-Yoon Lee, Dong-Jun Mun.
United States Patent |
10,889,876 |
Mun , et al. |
January 12, 2021 |
Non-heat treated wire rod having excellent cold workability and
manufactured method therefor
Abstract
Disclosed are a non-quenched and tempered wire rod and a
manufacturing method therefor, the non-quenched and tempered wire
rod comprising in percentage by weight: 0.15 to 0.30% of C; 0.05 to
0.3% of Si; 1.0 to 2.0% of Mn; 0.5% of less of Cr (except for 0%);
0.02% or less of P; 0.02% or less of S; 0.01 to 0.05% of sol. Al;
0.005 to 0.02% of Nb; 0.05 to 0.2% of V; 0.01% or less of N; Fe as
the remainder; and unavoidable impurities, wherein the non-quenched
and tempered wire rod satisfies the following formulas 1 and 2,
wherein, when the hardness of the wire rod measured in 1/2d
position and in 1/4d position in the diameter direction of the wire
rod is Hv,.sub.1/2d(Hv) and Hv,.sub.1/4d(Hv), respectively (here, d
is the diameter of the wire).
(Hv,.sub.1/2d+Hv,.sub.1/4d)/2.ltoreq.240 [Formula 1]
Hv,.sub.1/2d/Hv,.sub.1/4d.ltoreq.1.2 [Formula 2]
Inventors: |
Mun; Dong-Jun (Pohang-si,
KR), Lee; Sang-Yoon (Pohang-si, KR), Heo;
Yong-Kwan (Seoul, KR), Kim; Ha-Ni (Seoul,
KR) |
Applicant: |
Name |
City |
State |
Country |
Type |
POSCO |
Pohang-si |
N/A |
KR |
|
|
Assignee: |
POSCO (Pohang-si,
KR)
|
Family
ID: |
1000005295261 |
Appl.
No.: |
15/767,345 |
Filed: |
November 11, 2016 |
PCT
Filed: |
November 11, 2016 |
PCT No.: |
PCT/KR2016/013028 |
371(c)(1),(2),(4) Date: |
April 10, 2018 |
PCT
Pub. No.: |
WO2017/082687 |
PCT
Pub. Date: |
May 18, 2017 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20180298464 A1 |
Oct 18, 2018 |
|
Foreign Application Priority Data
|
|
|
|
|
Nov 12, 2015 [KR] |
|
|
10-2015-0158814 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C
38/04 (20130101); C21D 9/525 (20130101); C22C
38/02 (20130101); C22C 38/38 (20130101); C21D
8/065 (20130101); C21D 6/002 (20130101); C21D
6/005 (20130101); C22C 38/00 (20130101); C22C
38/001 (20130101); C22C 38/28 (20130101); C22C
38/24 (20130101); C22C 38/26 (20130101); C21D
6/008 (20130101); C22C 38/06 (20130101); C22C
38/002 (20130101); C21D 2211/009 (20130101); C21D
2211/005 (20130101) |
Current International
Class: |
C22C
38/00 (20060101); C22C 38/38 (20060101); C21D
6/00 (20060101); C21D 8/06 (20060101); C22C
38/02 (20060101); C22C 38/04 (20060101); C22C
38/06 (20060101); C22C 38/28 (20060101); C22C
38/26 (20060101); C22C 38/24 (20060101); C21D
9/52 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1270237 |
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1338528 |
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1468971 |
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1532300 |
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102071368 |
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103898417 |
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0922783 |
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1035222 |
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S62-074055 |
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H07-013257 |
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H10-195530 |
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2004-137542 |
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10-2001-0060772 |
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10-2009-0030544 |
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Feb 2010 |
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KR |
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10-2015-0022492 |
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Mar 2015 |
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Jun 2015 |
|
KR |
|
Other References
International Search Report dated Feb. 15, 2017 issued in
International Patent Application No. PCT/KR2016/013028 (with
English translation). cited by applicant .
Chinese Office Action dated Jul. 1, 2019 issued in Chinese Patent
Application No. 201680066249.X (with English translation). cited by
applicant.
|
Primary Examiner: Kastler; Scott R
Attorney, Agent or Firm: Morgan, Lewis & Bockius LLP
Claims
The invention claimed is:
1. A non-heat treated wire rod, comprising: carbon (C): 0.15 wt %
to 0.30 wt %, silicon (Si): 0.05 wt % to 0.3 wt %, manganese (Mn):
1.0 wt % to 2.0 wt %, chrome (Cr): 0.45 wt % or less (excluding
0%), phosphorus (P): 0.02 wt % or less, sulfur (S): 0.02 wt % or
less, soluble aluminum (sol. Al): 0.01 wt % to 0.05 wt %, niobium
(Nb): 0.005 wt % to 0.02 wt %, vanadium (V): 0.05 wt % to 0.2 wt %,
nitrogen (N): 0.008 wt % or less, iron (Fe) as a remainder; and
unavoidable impurities, wherein the non-heat treated wire rod
satisfies Formula 1 and Formula 2, when hardness of the wire rod,
measured in a 1/2d position and a 1/4d position in the diameter
direction of the wire rod, are Hv,.sub.1/2d(Hv) and
Hv,.sub.1/4d(Hv), respectively,
(Hv,.sub.1/2d+Hv,.sub.1/4d)/2.ltoreq.240 [Formula 1]
Hv,.sub.1/2d/Hv,.sub.1/4d.ltoreq.1.2 [Formula 2] where d is a
diameter of a wire rod.
2. The non-heat treated wire rod of claim 1, wherein the
unavoidable impurities include titanium (Ti), and are suppressed to
0.005 wt % or less of Ti.
3. The non-heat treated wire rod of claim 1, wherein the non-heat
treated wire rod includes a carbonitride having Nb and/or V, and an
average equivalent circular diameter of the carbonitride is 5 nm to
70 nm.
4. The non-heat treated wire rod of claim 3, wherein the number per
unit area of a carbonitride, of the carbonitride, having an average
equivalent circular diameter of 80 nm or more, is 5 per 1
.mu.m.sup.2 or less.
5. The non-heat treated wire rod of claim 1, wherein a carbon
equivalent (Ceq) is 0.5 or more and 0.6 or less.
6. The non-heat treated wire rod of claim 1, wherein the non-heat
treated wire rod satisfies Formula 3,
7.35[C]+1.88[Mn]+0.34[Cr]+0.25[Nb]+0.47[V].ltoreq.4.5 [Formula 3]
where each of [C], [Mn], [Cr], [Nb], and [V] is the content (%) of
a corresponding element.
7. The non-heat treated wire rod of claim 1, wherein the non-heat
treated wire rod satisfies Formula 4,
0.5.ltoreq.10[Nb]/[V].ltoreq.2.0 [Formula 4] where each of [Nb] and
[V] is the content (%) of a corresponding element.
8. The non-heat treated wire rod of claim 1, wherein the non-heat
treated wire rod includes ferrite and pearlite, as a
microstructure.
9. The non-heat treated wire rod of claim 1, including ferrite of
30 area % or more (excluding 100 area %) and pearlite of 70 area %
or less (excluding 0 area %), as a microstructure.
10. The non-heat treated wire rod of claim 8, wherein an average
grain size of the ferrite is 5 .mu.m to 25 .mu.m.
11. The non-heat treated wire rod of claim 1, wherein, during wire
drawing in a drawing amount (D) of 5% to 25%, hardness of the wire
rod after the wire drawing satisfies Formula 5,
Hv,.sub.1-10.ltoreq.(Hv,.sub.D,1/2d+Hv,.sub.D,1/4d)/2.ltoreq.Hv,.sub.1+10
[Formula 5] where Hv,.sub.1 is
"(Hv,.sub.1/2d+Hv,.sub.1/4d)/2+85.45.times.{1-exp(-D/11.41)}", and
Hv,.sub.D,1/2d and Hv,.sub.D,1/4d are hardness of the wire rod,
measured in a 1/2d position and a 1/4d position in the diameter
direction of the wire rod after the wire drawing, respectively.
12. A method for manufacturing a non-heat treated wire rod,
comprising: obtaining a billet by billet rolling after heating a
bloom at a heating temperature of 1200.degree. C. to 1300.degree.
C., the bloom including carbon (C): 0.15 wt % to 0.30 wt %, silicon
(Si): 0.05 wt % to 0.3 wt %, manganese (Mn): 1.0 wt % to 2.0 wt %,
chrome (Cr): 0.45 wt % or less (excluding 0%), phosphorus (P): 0.02
wt % or less, sulfur (S): 0.02 wt % or less, soluble aluminum (sol.
Al): 0.01 wt % to 0.05 wt %, niobium (Nb): 0.005 wt % to 0.02 wt %,
vanadium (V): 0.05 wt % to 0.2 wt %, nitrogen (N): 0.008 wt % or
less; iron (Fe) as a remainder; and unavoidable impurities, in
which a carbon equivalent (Ceq) is 0.5 or more and 0.6 or less, and
which satisfies Formula 3 and Formula 4; obtaining a wire rod by
wire rolling under the conditions of a finish rolling temperature
of Ae3.degree. C. to (Ae3+50.degree. C.), after reheating the
billet at a reheating temperature of 1050.degree. C. to
1250.degree. C.; and performing cooling, after winding the wire
rod, 7.35[C]+1.88[Mn]+0.34[Cr]+0.25[Nb]+0.47[V].ltoreq.4.5 [Formula
3] 0.5.ltoreq.10[Nb]/[V].ltoreq.2.0 [Formula 4] where each of [C],
[Mn], [Cr], [Nb], and [V] is the content (%) of a corresponding
element, and wherein, during the winding, a winding temperature is
750.degree. C. to 900.degree. C.
13. The method for manufacturing a non-heat treated wire rod of
claim 12, wherein the unavoidable impurities include titanium (Ti),
and are suppressed to 0.005 wt % or less of Ti.
14. The method for manufacturing a non-heat treated wire rod of
claim 12, wherein, during the heating of the bloom, retention time
at a heating temperature is 4 hours or more.
15. The method for manufacturing a non-heat treated wire rod of
claim 12, wherein, during the reheating of the billet, retention
time at a reheating temperature is 80 minutes or more.
16. The method for manufacturing a non-heat treated wire rod of
claim 12, wherein, during the cooling, a cooling rate is
0.1.degree. C./sec to 1.degree. C./sec.
Description
RELATED APPLICATIONS
This application is the U.S. National Phase under 35 U.S.C. .sctn.
371 of International Application No. PCT/KR2016/013028, filed on
Nov. 11, 2016 which in turn claims the benefit of Korean Patent
Application No. 10-2015-0158814 filed on Nov. 12, 2015, the
disclosures of which applications are incorporated by reference
herein.
TECHNICAL FIELD
The present disclosure relates to a non-quenched and tempered wire
rod having excellent cold workability and a method for
manufacturing the same, and more particularly, to a non-quenched
and tempered wire rod having excellent cold workability, suitable
for use as a material for vehicles or a material for machine
components, and a method for manufacturing the same.
BACKGROUND ART
A cold working method has effects of having excellent productivity
and a reduction in heat treatment costs, as compared to a hot
working method or a machine cutting method, and is thus widely used
for manufacturing machine components such as nuts, bolts, or the
like.
However, as described above, to manufacture such mechanical
components using a cold working method, excellent cold workability
of steel is essential. In detail, it is necessary for steel to have
low deformation resistance during cold working, and to have
excellent ductility. In this case, defective products may be
generated because the service life of a tool used during cold
working may be reduced if the deformation resistance of steel is
high, and because splitting may easily occur during cold working if
the ductility of steel is low.
Therefore, in the case of steel for cold working according to the
related art, a spheroidizing annealing heat treatment is performed
thereon before cold working. In this case, because, during the
spheroidizing annealing heat treatment, steel is softened,
deformation resistance is reduced, while ductility is improved, and
thus, cold workability is improved. However, in this case, because
additional costs may be incurred and manufacturing efficiency may
be reduced, development of a non-quenched and tempered wire rod
capable of securing excellent cold workability without the need for
an additional heat treatment has been required.
DISCLOSURE
Technical Problem
An aspect of the present disclosure may provide a non-quenched and
tempered wire rod in which excellent strength and cold workability
are able to be secured without an additional heat treatment and a
method for manufacturing the same.
Technical Solution
According to an aspect of the present inventive concept, a
non-quenched and tempered wire rod may include: carbon (C): 0.15 wt
% to 0.30 wt %, silicon (Si): 0.05 wt % to 0.3 wt %, manganese
(Mn): 1.0 wt % to 2.0 wt %, chrome (Cr): 0.5 wt % or less
(excluding 0%), phosphorus (P): 0.02 wt % or less, sulfur (S): 0.02
wt % or less, soluble aluminum (sol. Al): 0.01 wt % to 0.05 wt %,
niobium (Nb): 0.005 wt % to 0.02 wt %, vanadium (V): 0.05 wt % to
0.2 wt %, nitrogen (N): 0.01 wt % or less, iron (Fe) as a
remainder; and unavoidable impurities, wherein the non-quenched and
tempered wire rod satisfies Formula 1 and Formula 2, when hardness
of the wire rod, measured in a 1/2d position and a 1/4d position in
the diameter direction of the wire rod, are Hv,.sub.1/2d(Hv) and
Hv,.sub.1/4d(Hv), respectively,
(Hv,.sub.1/2d+Hv,.sub.1/4d)/2.ltoreq.240 [Formula 1]
Hv,.sub.1/2d/Hv,.sub.1/4d.ltoreq.1.2 [Formula 2]
where d is a diameter of a wire rod.
According to an aspect of the present inventive concept, a method
for manufacturing a non-quenched and tempered wire rod, may
include: obtaining a billet by billet rolling after heating a bloom
at a heating temperature of 1200.degree. C. to 1300.degree. C., the
bloom including carbon (C): 0.15 wt % to 0.30 wt %, silicon (Si):
0.05 wt % to 0.3 wt %, manganese (Mn): 1.0 wt % to 2.0 wt %, chrome
(Cr): 0.5 wt % or less (excluding 0%), phosphorus (P): 0.02 wt % or
less, sulfur (S): 0.02 wt % or less, soluble aluminum (sol. Al):
0.01 wt % to 0.05 wt %, niobium (Nb): 0.005 wt % to 0.02 wt %,
vanadium (V): 0.05 wt % to 0.2 wt %, nitrogen (N): 0.01 wt % or
less; iron (Fe) as a remainder; and unavoidable impurities, in
which a carbon equivalent (Ceq) is 0.5 or more and 0.6 or less, and
which satisfies Formula 3 and Formula 4;
obtaining a wire rod by wire rolling under the conditions of a
finish rolling temperature of Ae3.degree. C. to (Ae3+50).degree.
C., after reheating the billet at a reheating temperature of
1050.degree. C. to 1250.degree. C.; and
performing cooling, after winding the wire rod,
7.35[C]+1.88[Mn]+0.34[Cr]+0.25[Nb]+0.47[V].ltoreq.4.5 [Formula 3]
0.5.ltoreq.10[Nb]/[V].ltoreq.2.0 [Formula 4]
where each of [C], [Mn], [Cr], [Nb], and [V] is the content (%) of
a corresponding element.
Advantageous Effects
According to an exemplary embodiment in the present disclosure, a
non-quenched and tempered wire rod capable of sufficiently
suppressing deformation resistance during cold working, even when a
spheroidizing annealing heat treatment is omitted, may be
provided.
The various features, advantages, and effects of the present
disclosure are not limited to the above description, and can be
more easily understood while describing a specific embodiment of
the present disclosure.
BEST MODE FOR INVENTION
Hereinafter, a non-quenched and tempered wire rod having excellent
cold workability according to an aspect of the present disclosure
will be described in detail.
The present inventors have examined a wire rod from various aspects
to provide a wire rod capable of securing excellent cold
workability while having predetermined strength after wire drawing.
As a result, by appropriately controlling average hardness of a
wire rod and a hardness ratio of a center segregation portion and a
non-segregation portion of a wire rod, the present inventors have
found that a wire rod in which cold workability is not deteriorated
while having predetermined strength after wire drawing can be
provided, thereby completing the present disclosure.
A wire rod of the present disclosure satisfies Formula 1 and
Formula 2, when hardness of the wire rod measured in a 1/2d
position and a 1/4d position (here, d is a diameter of a wire rod)
in the diameter direction of the wire rod are Hv,.sub.1/2d(Hv) and
Hv,.sub.1/4d(Hv), respectively. If the wire rod does not satisfy
Formula 1, strength after wire drawing is significant, so cold
workability may be deteriorated. If the wire rod does not satisfy
Formula 2, cracking may occur in the wire rod during cold forging
after wire drawing. Thus, cold workability may be deteriorated.
(Hv,.sub.1/2d+Hv,.sub.1/4d)/2.ltoreq.240 [Formula 1]
Hv,.sub.1/2d/Hv,.sub.1/4d.ltoreq.1.2 [Formula 2]
In order to satisfy Formula 1 and Formula 2, a wire rod of the
present disclosure may have the following alloy composition and
composition range. It is noted in advance that the content of each
element described below is based on weight, unless otherwise
specified.
First, an alloy composition and composition range of a non-quenched
and tempered wire rod will be described in detail.
Carbon (C): 0.15% to 0.30%
Carbon serves to increase the strength of a wire rod. In the
present disclosure, in order to realize the effect described above,
carbon is preferably included in an amount of 0.15% or more, and
more preferably, included in an amount of 0.16% or more. However,
if the content of carbon is excessive, deformation resistance of
steel may rapidly increase, and thus, a problem in which cold
workability is deteriorated may occur. Thus, an upper limit of the
content of carbon is preferably 0.3%, more preferably 0.29%.
Silicon (Si): 0.05% to 0.3%
Silicon is an element useful as a deoxidizer. In the present
disclosure, in order to realize the effect described above, silicon
is preferably included in an amount of 0.05% or more. However, if
the content of silicon is excessive, deformation resistance of
steel may rapidly increase through solid solution strengthening,
and thus, a problem in which cold workability is deteriorated may
occur. Thus, an upper limit of the content of silicon is preferably
0.3%, more preferably 0.25%.
Manganese (Mn): 1.0% to 2.0%
Manganese is an element useful as a deoxidizer and a desulfurizing
agent. In the present disclosure, in order to realize the effect
described above, manganese is preferably included in an amount of
1.0% or more, and more preferably, included in an amount of 1.1% or
more. However, if the content of manganese is excessive, the
strength of steel itself is significantly increased, and thus, a
problem in which cold workability is deteriorated may occur. Thus,
an upper limit of the content of manganese is preferably 2.0%, more
preferably 1.8%.
Chromium (Cr): 0.5% or Less (Excluding 0%)
Chromium serves to promote transformation of ferrite and pearlite
during hot rolling. In addition, while the strength of steel itself
is not increased more than necessary, a carbide in steel is
precipitated and an amount of solid carbon is reduced, thereby
contributing to a reduction in dynamic deformation aging caused by
solid carbon. However, if the content of chromium is excessive, the
strength of steel itself is significantly increased, so deformation
resistance of steel rapidly increases. Thus, a problem in which
cold workability is deteriorated may occur. The content of chromium
is preferably 0.5% or less (excluding 0%), more preferably 0.05% to
0.45%.
Phosphorus (P): 0.02% or Less
Phosphorus, an impurity which is inevitably contained, is
segregated in grain boundaries to reduce toughness of steel, and is
an element mainly responsible for a decrease in delayed fracture
resistance. Thus, the content of phosphorus is preferably
controlled to be as low as possible. Theoretically, it is
advantageous to control the content of phosphorus to be 0%, but
phosphorus is inevitably contained in a manufacturing process.
Thus, it is important to manage an upper limit of phosphorus. In
the present disclosure, the upper limit of the content of
phosphorus is managed to be 0.02%.
Sulfur (S): 0.02% or Less
Sulfur, an impurity which is inevitably contained, is segregated in
grain boundaries to significantly reduce ductility, and is an
element mainly responsible for a deterioration of cold
forgeability, delayed fracture resistance and stress relaxation
characteristics by forming sulfide (an MnS inclusion) in steel.
Thus, the content of sulfur is preferably controlled to be as low
as possible. Theoretically, it is advantageous to control the
content of sulfur to be 0%, but sulfur is inevitably contained in a
manufacturing process. Thus, it is important to manage an upper
limit of sulfur. In the present disclosure, the upper limit of the
content of sulfur is managed to be 0.02%, more preferably 0.01%,
further more preferably 0.009%, most preferably 0.008%.
Soluble Aluminum (Sol. Al): 0.01% to 0.05%
Soluble aluminum is an element useful as a deoxidizer. In the
present disclosure, in order to realize the effect described above,
soluble aluminum is preferably included in an amount of 0.01% or
more, more preferably, included in an amount of 0.015% or more, and
further more preferably, included in an amount of 0.02% or more.
However, if the content of soluble aluminum exceeds 0.05%, by the
formation of AlN, an austenite grain refinement effect is
increased, so cold workability may be lowered. Thus, in the present
disclosure, an upper limit of the content of soluble aluminum is
managed to be 0.05%.
Niobium (Nb): 0.005% to 0.02%
Niobium, an element serving to limit movement of austenite and
ferrite to a grain boundary by forming a carbonitride, is included
in an amount of 0.005% or more. However, the carbonitride acts as a
point of fracture, and thus may reduce impact toughness, in detail,
low temperature impact toughness. Thus, niobium is preferably added
within a solubility limit. Furthermore, if the content of niobium
is excessive, a problem in which a concentration exceeds a solid
solution limit and a coarse precipitate is formed may occur. Thus,
the content of niobium is preferably limited to 0.02% or less, more
preferably to 0.018% or less.
Vanadium (V): 0.05% to 0.2%
Vanadium, an element serving to limit movement of austenite and
ferrite to a grain boundary by forming a carbonitride, in a manner
similar to niobium, is included in an amount of 0.05% or more.
However, the carbonitride acts as a point of fracture, and thus may
reduce impact toughness, in detail, low temperature impact
toughness. Thus, vanadium is preferably added within a solubility
limit. Thus, the content of vanadium is preferably limited to 0.2%
or less, more preferably to 0.18% or less.
Nitrogen (N): 0.01% or Less
Nitrogen is an impurity which is inevitably contained. If the
content of nitrogen is excessive, an amount of solid nitrogen
increases, so deformation resistance of steel rapidly increases.
Thus, a problem in which cold workability is deteriorated may
occur. Theoretically, it is advantageous to control the content of
nitrogen to be 0%, but nitrogen is inevitably contained in a
manufacturing process. Thus, it is important to manage an upper
limit of nitrogen. In the present disclosure, the upper limit of
the content of nitrogen is managed to be 0.01%, more preferably
managed to be 0.008%, further more preferably managed to be
0.007%.
The remainder of an alloy composition is iron (Fe). In addition,
the non-quenched and tempered wire rod of the present disclosure
may include other impurities which may be included in an industrial
production process of steel according to the related art. These
impurities may be known to any person skilled in the art, and
therefore the type and content of the impurities are not
particularly limited in the present disclosure.
However, since titanium (Ti) corresponds to a representative
impurity, a content of which is to be suppressed in order to obtain
the effect of the present disclosure, a brief description thereof
will be provided below.
Titanium (Ti): 0.005% or Less
Titanium, a carbonitride forming element, may form a carbonitride
at a higher temperature, as compared to Nb and V. If titanium is
included in steel, it may be advantageous to fix C and N. However,
in this case, Nb and/or V is precipitated using Ti carbonitrides as
a nucleus, and thus a large amount of coarse carbonitrides are
formed in a base, so cold workability may be deteriorated. Thus, it
is important to manage an upper limit of titanium. In the present
disclosure, the upper limit of the content of titanium is
preferably managed to be 0.005%, more preferably managed to be
0.004%.
For example, a carbon equivalent (Ceq) of a wire rod of the present
disclosure may be 0.5 or more and 0.6 or less. Here, the carbon
equivalent (Ceq) may be defined by Equation 1. If the carbon
equivalent (Ceq) is less than 0.5 or exceeds 0.6, it may be
difficult to secure target strength. Ceq=[C]+[Si]/9+[Mn]/5+[Cr]/12
[Equation 1]
Where, each of [C], [Si], [Mn], and [Cr] refers to the content (%)
of a corresponding element.
For example, the contents of C, Mn, Cr, Nb, and V may satisfy
Formula 3, If the contents thereof do not satisfy Formula 3, by
segregation in a center portion, a difference in hardness between a
center segregation portion and a non-segregation portion of a wire
rod rapidly increases, so possibility of internal cracking during a
cold forging process rapidly increases. Thus, cold workability may
be deteriorated.
7.35[C]+1.88[Mn]+0.34[Cr]+0.25[Nb]+0.47[V].ltoreq.4.5 [Formula
3]
Where, each of [C], [Mn], [Cr], [Nb], and [V] refers to the content
(%) of a corresponding element.
For example, the contents of Nb and V may satisfy Formula 4. The
inventors confirmed that formation of coarse Nb and V composite
carbonitrides was suppressed, when the contents of Nb and V satisfy
Formula 4. If the contents of Nb and V do not satisfy Formula 4, Nb
and V carbonitrides are not sufficiently solidified during billet
reheating and are coarsely precipitated in a base during a wire rod
manufacturing process, so cold workability may be deteriorated. A
lower limit of a value of 10[Nb]/[V] is more preferably 0.6,
further more preferably 0.7. An upper limit of a value of
10[Nb]/[V] is more preferably 1.5, further more preferably 1.2.
0.5.ltoreq.10[Nb]/[V].ltoreq.2.0 [Formula 4]
Where, each of [Nb] and [V] refers to the content (%) of a
corresponding element.
For example, the non-quenched and tempered wire rod includes a
carbonitride including Nb and/or V, and an average equivalent
circular diameter of the carbonitride may be 70 nm or less. If the
average equivalent circular diameter of the carbonitride exceeds 70
nm, the carbonitrides may act as a point of fracture at a center
segregation portion. Here, the carbonitride refers to a precipitate
including carbon and/or nitrogen.
For example, the number per unit area of a carbonitride in which an
average equivalent circular diameter is 80 nm or more, of the
carbonitride including Nb and/or V, may be 5 per 1 .mu.m.sup.2 or
less. If the number per unit area of the carbonitride in which an
average equivalent circular diameter is 80 nm or more exceeds 5 per
1 .mu.m.sup.2, it may be difficult to secure target cold
workability.
Meanwhile, in the present disclosure, a method of measuring an
average equivalent circular diameter of the carbonitride including
Nb and/or V is not particularly limited, but the following method
may be used by way of example. A non-quenched and tempered wire rod
may be cut in a direction perpendicular to a longitudinal
direction, and then an image of a cross-section may be captured at
.times.1,000 magnification using a Field Emission Scanning Electron
Microscope (FE-SEM) in a 1/4d position (here, d refers to a
diameter of a non-quenched and tempered wire rod), and a
composition of each precipitate is analyzed using an Electron Probe
Micro-Analyzer (EPMA), and a type thereof is classified. Then, the
type thereof is analyzed, and thus, the number of a coarse
carbonitride in which an average equivalent circular diameter is 80
nm or more, of a carbonitride including Nb and/or V, can be
calculated.
For example, a wire rod of the present disclosure may include
ferrite and pearlite as a microstructure, more preferably, ferrite
of 30% or more (excluding 100%) and pearlite of 70% or less
(excluding 0%) in an area fraction. When the structure described
above is secured, it has the advantage of securing excellent cold
workability and securing excellent strength after a proper wire
drawing.
In addition, for example, an average grain size of ferrite may be 5
.mu.m to 25 .mu.m, more preferably 10 .mu.m to 20 .mu.m. If an
average grain size of ferrite is less than 5 .mu.m, due to grain
refinement, strength increases, cold workability may be reduced. On
the other hand, the average grain size of ferrite exceeds 25 .mu.m,
strength may decrease.
In addition, for example, a standard deviation of a grain size of
ferrite may be 5 .mu.m or less (including 0 .mu.m), more preferably
3 .mu.m or less (including 0 .mu.m). If the standard deviation of a
grain size of ferrite exceeds 5 .mu.m, coarse ferrite becomes a
point of brittle fracture, so toughness and workability of steel
may be deteriorated.
Meanwhile, an average grain size and a standard deviation of a
grain size of pearlite, formed together with ferrite, is not
particularly limited, because the average grain size and the
standard deviation of a grain size of pearlite are influenced by
the average grain size and the standard deviation of a grain size
of ferrite. Here, a grain size refers to an equivalent circular
diameter of particles detected by observing a cross-section in a
longitudinal direction of a wire rod.
For example, a wire rod of the present disclosure has an advantage
of having excellent ductility with a cross-section reduction rate
(RA) of 70% or more in a state of a wire rod.
For example, when a wire rod of the present disclosure is drawn in
a drawing amount (D) of 5% to 25%, hardness of the wire rod after
wire drawing may satisfy Formula 5. If the hardness of the wire rod
after wire drawing does not satisfy Formula 5, an increase in
strength caused by work hardening is significant, so cold
workability may rapidly decrease.
Hv,.sub.1-10.ltoreq.(Hv,.sub.D,1/2d+Hv,.sub.D,1/4d)/2.ltoreq.Hv,.sub.1+10
[Formula 5]
Where, Hv,.sub.1 refers to
"(Hv,.sub.1/2D+Hv,.sub.1/4D)/2+85.45.times.{1-exp(-D/11.41)}", and
Hv,.sub.D,1/2d and Hv,.sub.D,1/4d refer to hardness of the wire rod
measured in a 1/2d position and a 1/4d position in the diameter
direction of the wire rod after wire drawing, respectively.
The wire rod of the present disclosure for drawing described above
may be manufactured in various methods, and a method for
manufacturing the same is not particularly limited. However, as an
exemplary example, the wire rod may be manufactured by the
following method.
Hereinafter, a method for manufacturing a non-quenched and tempered
wire rod having excellent cold workability, another aspect of the
present disclosure, will be described in detail.
First, a bloom satisfying the composition is heated, and is then
billet-rolled to obtain a billet.
A heating temperature of the bloom is preferably 1200.degree. C. to
1300.degree. C., more preferably 1220.degree. C. to 1280.degree. C.
If the heating temperature of the bloom is lower than 1200.degree.
C., hot deformation resistance may be increased. On the other hand,
if the heating temperature of the bloom exceeds 1300.degree. C., by
coarsening of austenite, ductility may be deteriorated.
For example, when the bloom is heated, the retention time at
heating temperature may be equal to 4 hours or more. If the
retention time is less than 4 hours, a homogenization treatment may
be insufficient. Meanwhile, when the retention time at heating
temperature is longer, homogenization may be advantageously
performed, so a segregation may be easily reduced. In the present
disclosure, an upper limit of the retention time is not
particularly limited.
Next, the billet is reheated, and is then wire rolled to obtain a
non-quenched and tempered wire rod.
A reheating temperature of the billet is preferably 1050.degree. C.
to 1250.degree. C., more preferably 1100.degree. C. to 1200.degree.
C. If the reheating temperature of the billet is less than
1050.degree. C., hot deformation resistance is increased, so
productivity may be reduced. On the other hand, if the heating
temperature exceeds 1250.degree. C., a ferrite grain may be
significantly coarse, so ductility may be lowered.
For example, when the billet is reheated, the retention time at
reheating temperature may be equal to 80 minutes or more. If the
retention time is mess than 80 minutes, a homogenization treatment
may be insufficient. Meanwhile, in the case that the retention time
at reheating temperature is longer, homogenization of segregation
promoting elements may be advantageously performed. In the present
disclosure, an upper limit of the retention time is not
particularly limited.
During wire rolling, a finish rolling temperature is preferably
Ae3.degree. C. to (Ae3+50).degree. C. If the finish rolling
temperature is less than Ae3.degree. C., due to a temperature
deviation of a center portion and a surface part of a wire rod, a
size deviation of the particles of a ferrite grain may occur. Due
to an increase in strength by ferrite grain refinement, deformation
resistance may be increased. On the other hand, if the finish
rolling temperature exceeds Ae3+50.degree. C., a ferrite grain is
significantly coarse, so toughness may be lowered. For reference,
Ae3 can be calculated from Equation 2. For reference, here, a
finish rolling temperature refers to a surface temperature of a
slab at a finish rolling start point, and a surface temperature of
the slab after finish rolling starts may be increased more than the
finish rolling temperature due to a heat effect. In the present
disclosure, the surface temperature of the slab after finish
rolling starts is not particularly limited. Ae3(.degree.
C.)=930-185 [C]+60[Si]-25[Mn]-500[P]+12[Cr]-200[Al]+100[V]-400[Ti]
[Equation 2]
Where, each of [C], [Si], [Mn], [P], [Cr], [Al], [V], and [Ti]
refers to the content (%) of a corresponding element.
Thereafter, the non-quenched and tempered wire rod is wound, and is
then cooled.
A winding temperature of a non-quenched and tempered wire rod may
be 750.degree. C. to 900.degree. C., more preferably 800.degree. C.
to 850.degree. C. If the winding temperature is less than
750.degree. C., martensite in a surface layer, generated during
cooling, is not recovered by heat recuperation, while tempered
martensite is generated, so steel becomes hard and brittle. Thus,
cold workability may be lowered. On the other hand, if the winding
temperature exceeds 900.degree. C., a thick scale is formed on a
surface, so trouble may easily occur during descaling, and the
cooling time is longer and thus productivity may be lowered.
A cooling rate during cooling of a non-quenched and tempered wire
rod may be 0.1.degree. C./sec to 1.degree. C./sec, preferably
0.3.degree. C./sec to 0.8.degree. C./sec. In this case, the cooling
rate described above is provided to stably form a ferrite and
pearlite composite structure. If the cooling rate is less than
0.1.degree. C./sec, lamellar spacing in a pearlite structure is
widened, so ductility may be insufficient. If the cooling rate
exceeds 1.degree. C./sec, a ferrite fraction may be insufficient,
so cold workability may be deteriorated.
MODE FOR INVENTION
Hereinafter, the present disclosure will be described in more
detail through examples. However, the description of these examples
is for the purpose of illustrating the practice of the present
disclosure, and the present disclosure is not limited by the
description of these examples. The scope of the present disclosure
is determined by the matters described in the appended claims and
the matters reasonably deduced therefrom.
A bloom having the composition described in Table 1 was heated at
1250.degree. C. for 5 hours, and was then billet rolled under a
finish rolling temperature condition of 1150.degree. C. to obtain a
billet. Thereafter, the billet was reheated at 1150.degree. C. for
2 hours, and was then wire rolled with a wire diameter of 20 mm to
manufacture a non-quenched and tempered wire rod. In the case of
Comparative Example 1, finish rolling was performed at a finish
rolling temperature of 770.degree. C. In the case of other
Examples, finish rolling was performed at a finish rolling
temperature of 850.degree. C. Thereafter, winding was performed at
a temperature of 800.degree. C., and cooling was performed at a
rate of 0.5.degree. C./sec. Thereafter, a microstructure of the
wire rod, having been cooled, was observed using a FE-SEM, and an
equivalent circular diameter of a carbonitride, and the like were
calculated, and then hardness was measured in a 1/2d position and a
1/4d position in the diameter direction of the wire rod. A result
thereof is illustrated in Table 2.
In addition, cold workability of a wire rod, having been cooled,
was evaluated, and is illustrated in Table 2. A notch compression
specimen was subjected to a compression test in which true strain
is 0.7, and cold workability was evaluated considering whether
cracking occurred. If cracking did not occur, cold workability was
evaluated as "GO". If cracking occurred, cold workability was
evaluated as "NG".
TABLE-US-00001 TABLE 1 ALLOY COMPOSITION (WT %) sol. STEEL C Si Mn
P S Cr Al Nb V N Ti {circle around (1)}* {circle around (2)}**
{circle around (3)}*** INVENTIVE 0.16 0.16 1.45 0.011 0.0042 0.41
0.03 0.009 0.12 0.0045 0.003 0.- 50 4.10 0.75 STEEL 1 INVENTIVE
0.18 0.17 1.41 0.010 0.0055 0.35 0.02 0.012 0.15 0.0044 0.004 0.-
51 4.17 0.8 STEEL 2 INVENTIVE 0.19 0.18 1.38 0.012 0.0053 0.31 0.04
0.010 0.11 0.0042 0.001 0.- 51 4.15 0.91 STEEL 3 INVENTIVE 0.21
0.14 1.42 0.011 0.0061 0.25 0.03 0.011 0.13 0.0053 0.002 0.- 53
4.36 0.85 STEEL 4 INVENTIVE 0.24 0.17 1.37 0.012 0.0043 0.23 0.04
0.009 0.11 0.0052 0.003 0.- 55 4.47 0.82 STEEL 5 INVENTIVE 0.27
0.18 1.26 0.011 0.0057 0.16 0.03 0.008 0.10 0.0048 0.002 0.- 56
4.46 0.80 STEEL 6 INVENTIVE 0.28 0.21 1.20 0.010 0.0052 0.14 0.02
0.009 0.08 0.0040 0.004 0.- 56 4.40 1.12 STEEL 7 INVENTIVE 0.29
0.19 1.17 0.011 0.0064 0.13 0.03 0.008 0.07 0.0037 0.001 0.- 56
4.44 1.14 STEEL 8 INVENTIVE 0.19 0.21 1.59 0.011 0.0056 0.18 0.03
0.006 0.11 0.0054 0.005 0.- 54 4.49 0.55 STEEL 9 COMPARATIVE 0.15
0.18 1.75 0.010 0.0055 0.21 0.03 0.007 0.17 0.0055 0.015 - 0.54
4.55 0.41 STEEL 1 COMPARATIVE 0.17 0.17 1.68 0.012 0.0062 0.19 0.02
0.006 0.13 0.0053 0.012 - 0.54 4.54 0.45 STEEL 2 COMPARATIVE 0.18
0.15 1.63 0.013 0.0053 0.23 0.04 0.002 0.20 0.0047 0.010 - 0.54
4.56 0.1 STEEL 3 COMPARATIVE 0.22 0.16 1.59 0.010 0.0062 0.17 0.03
0.007 0.16 0.0045 0.008 - 0.57 4.74 0.44 STEEL 4 COMPARATIVE 0.26
0.17 1.52 0.011 0.0063 0.25 0.04 0.005 0.12 0.0052 0.007 - 0.61
4.91 0.42 STEEL 5 COMPARATIVE 0.28 0.18 1.48 0.012 0.0051 0.27 0.02
0.002 0.07 0.0046 0.004 - 0.62 4.97 0.29 STEEL 6 COMPARATIVE 0.32
0.20 1.32 0.011 0.0059 0.29 0.03 0.002 0.06 0.0048 0.003 - 0.63
4.96 0.33 STEEL 7 COMPARATIVE 0.35 0.22 1.24 0.010 0.0047 0.31 0.02
0.001 0.05 0.0054 0.006 - 0.65 5.03 0.2 STEEL 8 *{circle around
(1)} = [C] + [Si]/9 + [Mn]/5 + [Cr]/12 **{circle around (2)} =
7.35[C] + 1.88[Mn] + 0.34[Cr] + 0.25[Nb] + 0.47[V], ***{circle
around (3)} = 10[Nb]/[V]
TABLE-US-00002 TABLE 2 FERRITE FERRITE GRAIN FERRITE AVERAGE SIZE
STANDARD MICRO- FRACTION GRAIN DEVIATION COLD STEEL STRUCTURE*
(AREA %) SIZE (.mu.m) (.mu.m) {circle around (1)}** {circle around
(2)}*** WORKABILITY NOTE INVENTIVE F + P 55.4 14 1.9 214.2 1.04 GO
INVENTIVE STEEL 1 EXAMPLE 1 INVENTIVE F + P 52.8 13 2.1 230.0 1.08
GO INVENTIVE STEEL 2 EXAMPLE 2 INVENTIVE F + P 51.5 16 2.4 220.4
1.07 GO INVENTIVE STEEL 3 EXAMPLE 3 INVENTIVE F + P 48.7 11 2.8
231.4 1.10 GO INVENTIVE STEEL 4 EXAMPLE 4 INVENTIVE F + P 46.8 12
2.2 233.2 1.18 GO INVENTIVE STEEL 5 EXAMPLE 5 INVENTIVE F + P 43.6
11 3.0 230.8 1.17 GO INVENTIVE STEEL 6 EXAMPLE 6 INVENTIVE F + P
43.1 10 2.7 229.9 1.12 GO INVENTIVE STEEL 7 EXAMPLE 7 INVENTIVE F +
P 42.3 9 2.9 229.2 1.13 GO INVENTIVE STEEL 8 EXAMPLE 8 INVENTIVE F
+ P 51.3 8 6.8 251.5 1.31 GO COMPARATIVE STEEL 9 EXAMPLE 1
COMPARATIVE F + P 56.7 18 5.1 236.9 1.23 GO COMPARATIVE STEEL 1
EXAMPLE 2 COMPARATIVE F + P 53.2 19 5.4 231.2 1.22 GO COMPARATIVE
STEEL 2 EXAMPLE 3 COMPARATIVE F + P 52.5 17 5.3 246.7 1.24 GO
COMPARATIVE STEEL 3 EXAMPLE 4 COMPARATIVE F + P 48.1 13 5.6 253.0
1.29 GO COMPARATIVE STEEL 4 EXAMPLE 5 COMPARATIVE F + P 44.0 14 5.5
241.6 1.34 GO COMPARATIVE STEEL 5 EXAMPLE 6 COMPARATIVE F + P 43.4
15 5.2 238.4 1.37 GO COMPARATIVE STEEL 6 EXAMPLE 7 COMPARATIVE F +
P 38.7 16 6.1 242.6 1.35 GO COMPARATIVE STEEL 7 EXAMPLE 8
COMPARATIVE F + P 34.1 12 5.4 248.3 1.41 GO COMPARATIVE STEEL 8
EXAMPLE 9 *F REFERS TO FERRITE, AND P REFERS TO PEARLITE **{circle
around (1)} = (Hv.sub.,1/2d + Hv.sub.,1/4d)/2 ***{circle around
(2)} = Hv.sub.,1/2d/Hv.sub.,1/4d
Thereafter, a drawing amount of each of 10%, 20%, and 30% was
applied to each wire rod, and a steel wire was manufactured. A
hardness of each steel wire, having been manufactured, was measured
in a 1/2d position and a 1/4d position in the diameter direction,
and cold workability was evaluated. A result thereof is illustrated
in Table 3.
TABLE-US-00003 TABLE 3 (Hv.sub.,D,1/2d + Hv.sub.,D,1/4d)/2 COLD
WORKABILITY STEEL 10% 20% 30% 10% 20% 30% NOTE INVENTIVE 264.0
277.2 286.0 GO GO GO INVENTIVE STEEL 1 EXAMPLE 1 INVENTIVE 279.8
293.0 301.8 GO GO GO INVENTIVE STEEL 2 EXAMPLE 2 INVENTIVE 270.2
283.4 292.2 GO GO GO INVENTIVE STEEL 3 EXAMPLE 3 INVENTIVE 281.3
294.5 303.3 GO GO GO INVENTIVE STEEL 4 EXAMPLE 4 INVENTIVE 283.1
296.3 305.1 GO GO GO INVENTIVE STEEL 5 EXAMPLE 5 INVENTIVE 280.7
293.9 302.7 GO GO GO INVENTIVE STEEL 6 EXAMPLE 6 INVENTIVE 279.7
293.0 301.8 GO GO GO INVENTIVE STEEL 7 EXAMPLE 7 INVENTIVE 279.1
292.3 301.1 GO GO GO INVENTIVE STEEL 8 EXAMPLE 8 INVENTIVE 293.5
310.4 321.6 GO GO NG COMPARATIVE STEEL 9 EXAMPLE 1 COMPARATIVE
288.8 292.4 312.3 GO GO NG COMPARATIVE STEEL 1 EXAMPLE 2
COMPARATIVE 285.1 294.7 304.3 GO GO NG COMPARATIVE STEEL 2 EXAMPLE
3 COMPARATIVE 290.6 308.2 318.7 GO GO NG COMPARATIVE STEEL 3
EXAMPLE 4 COMPARATIVE 299.2 311.5 322.9 GO GO NG COMPARATIVE STEEL
4 EXAMPLE 5 COMPARATIVE 297.5 302.1 313.5 GO NG NG COMPARATIVE
STEEL 5 EXAMPLE 6 COMPARATIVE 288.7 293.9 309.2 GO NG NG
COMPARATIVE STEEL 6 EXAMPLE 7 COMPARATIVE 290.6 307.1 316.6 GO NG
NG COMPARATIVE STEEL 7 EXAMPLE 8 COMPARATIVE 296.1 306.8 318.1 GO
NG NG COMPARATIVE STEEL 8 EXAMPLE 9
As can be seen from Table 3, in the case of Inventive Examples 1 to
8 satisfying an alloy composition and manufacturing conditions
proposed in the present disclosure, an average hardness of a wire
rod and a hardness ratio of a center segregation portion and a
non-segregation portion of a wire rod satisfy a range proposed in
the present disclosure and it can be seen that cold workability is
excellent. On the other hand, in the case of Comparative Examples 1
to 9, a hardness ratio of a center segregation portion and a
non-segregation portion of a wire rod exceeds a range proposed in
the present disclosure. Thus, cracking occurred inside during cold
forging after wire drawing, and cold workability was inferior, as
compared to an Inventive Steel.
* * * * *